Tutorial: Triggered updates are sent as soon as a change in network topology is discovered. This speeds convergence and helps prevent problems caused by out-of-date information. Triggered updates are combined with holddown timers to make sure new routing information is propagated to the entire network by the triggered updates.

Split horizon stops routing loops by preventing route-update information from being sent back out of the interface on which it arrived. Routing loops occur when there is slow convergence on a network, which causes inconsistent routing entries. These routing loops lead to excessive bandwidth being used and can cripple the network.

Poison reverse “poisons” a failed route by increasing its cost to infinity. This is needed only for very large loops, because small ones can be prevented by split horizon, holddown timers, and triggered updates.

Holddown timers prevent routers from making a change that will affect routes for a specified period of time. The holddown time is set to be slightly greater than the time required to update the entire network.